Specific thrust in a jet engine is represented by pounds of thrust per pound of airflow. It is advantageous to have a high specific thrust for jet engines which propel aircraft at supersonic velocities, so as to provide for relatively small engine size. In order to achieve such high specific thrust, high airflows or high exhaust velocities are often used. However, jet noise is directly proportional to the exponential values of exhaust velocities, and for a high speed civil transport aircraft operable at flight velocities of greater than about Mach 2, jet noise is proportional to the fourth order of exhaust velocity. One of the major sources of noise from the supersonic exhaust is Mach waves which are generated when the air flow exceeds supersonic speeds. The high air velocities from the exhaust result in the generation of supersonic turbulent eddies which create pressure waves or noise.
Government regulations often limit the acceptable amount of noise which may be generated by an aircraft engine during the takeoff, initial climb and landing of the aircraft. Thus, in the development of supersonic aircraft, it has become increasingly important to find a means of controlling jet engine noise including Mach waves.
The main devices currently considered for suppression of Mach waves are lobe mixers and ejector shrouds. Lobe mixers accelerate the mixing between the jet gas and surrounding air causing the jet exhaust to decelerate faster as it exits the nozzle, thereby reducing the supersonic speed of the jet exhaust. Although this mixing does achieve a reduction in noise level, the mixing also results in significant thrust losses.
Ejector shrouds are a commonly considered alternative to lobe mixers for reducing engine noise. Ejector shrouds surround the jet core and act as mufflers. While such shrouds are effective at low flight speeds, the shrouds adversely affect aircraft performance at high speeds. Thus shrouds are generally designed to be retractable and storable at high speeds. However retractable or storable shrouds require complex mechanisms. Such complex mechanisms cause severe weight and space penalties.
Thus, there remains a need for a relatively simple mechanism which reduces supersonic jet engine noise without the severe thrust losses or severe weight and space penalties experienced by the prior art.